The present invention relates to a pneumatically operated disk brake which is provided particularly for commercial road vehicles.
Pneumatically operated or pneumatic disk brakes of the above-mentioned type are known, for example, from German Patent Document DE-OS 37 16 202, German Patent Document DE-OS 40 32 885 as well as German Patent Document DE-OS 42 12 384 which is not a prior publication. In the case of these known disk brakes, a caliper, which can be displaced in the axial direction, reaches around a brake disk. A pneumatically operated brake application device is arranged on one side of the caliper and during the actuating of this brake application device, a brake shoe, which is situated on this side of the brake disk, is pressed against the corresponding friction surface of the brake disk. The caliper is displaced in the opposite direction because of the reaction forces and, as a result, a brake shoe situated on the opposite side, also presses against the brake disk.
In the case of these known disk brakes, the brake application device has a rotary lever as a control element which is disposed to swivel about a swivel axis which extends parallel to the plane of the brake disk. In this case, the rotary lever, on its side facing the brake disk, rests, by means of an eccentric connection, approximately in the longitudinal center on a traverse member which extends parallel to the swivel axis. The traverse member is guided to be displaceable with respect to the brake disk. At least one adjusting spindle provided with an external thread is adjustably screwed in a respectively assigned internal thread of the traverse member. According to the number of adjusting spindles, this type of a brake application device is indicated to be either a "single-spindle type" or a "double-spindle type".
For operating the rotary lever, a compressed-air cylinder, when acted upon by compressed air, displaces an outlet-side plunger rod by a certain distance. Since the end of the plunger rod engages, by a ball socket bearing, in an articulated manner with the end of the lever arm of the rotary lever, the rotary lever is swivelled by a corresponding extent. The rotary lever, by means of the eccentric, as explained above, displaces the traverse member in the direction of the brake disk. In the known brake application devices, the above-mentioned ball socket bearing is formed by a socket in the shape of a dome-shaped recess constructed on the end of the lever arm of the rotary lever. The recess receives the end of the plunger rod which is shaped as a hemisphere.
With a view to an efficiency of the brake application device that should be as high as possible as well as, in particular, and also taking into account the optimization of its hysteresis characteristics, the known brake application devices provide a grease lubrication on the contact surface between the socket and the hemisphere during the operation. This achieves the desired operating characteristics by the corresponding reduction of the surface friction and, at the same time, also reduce wear. Tests on the initially mentioned brake application devices have shown, for example, that by reducing the coefficients of friction of the lever mechanism, a clear improvement of the hysteresis action of the brake can be achieved. Furthermore, it was already taken into consideration to harden the end of the plunger rod which is shaped as a hemisphere in order to further reduce wear.
However, in practice, the grease lubrication presents problems because a lubrication for the length of the service life can hardly be implemented in the case of the normal operating conditions. Relubrication, which becomes necessary from time to time, requires a demounting of the brake application device which, however, results in high shop costs. The mentioned hardening of the end of the plunger rod also considerably increases the manufacturing costs of the compressed-air cylinder. For this reason, this measure is also considered to be disadvantageous.
In order to avoid the above-mentioned disadvantages of the conventional ball socket bearing between the plunger rod and the rotary lever, it may in principle be considered to provide a slide bearing. However, experiments made by the applicant using such bearings have not yet had any satisfactory results. In particular, it has so far not been possible to find a bearing which has such low coefficients of friction that the achievable hysteresis is satisfactory. The use of a slide bearing has also been rejected because the bearing stress occurring in the case of the preferred geometry of the lever arrangement has exceeded the maximal loading capacity of the tested slide bearings so that a sufficiently high service life could not be achieved.
The invention is based on the object of further developing a pneumatically operated disk brake in such a manner that the manufacturing and maintenance costs of the brake application device can be clearly reduced while the brake hysteresis remains good.
According to the invention, a slide bearing is arranged between the ball and the socket of the ball socket bearing, wherein the sliding surface of the slide bearing consists of at least 70% by volume polyvinylidenefluoride (PVDF), at least 10% by volume polytetrafluoro ethylene (PTFE), and a remainder of lead. Experiments have surprisingly shown that a slide bearing having the mentioned composition, in the case of the dynamic loads occurring during the operation of the lever mechanism and of the ball socket bearing, will not fail and will always have a sufficient, extremely long service life even under extremely high load values. Furthermore, the above-mentioned slide bearing has the characteristic that, in the highly stressed condition, its coefficient of friction is reduced significantly. The coefficients of friction achieved, according to the invention, reach or even exceed those of a roller bearing, and the coefficients of friction of the conventional grease lubrication are exceeded in every case.
Since a slide bearing of this type is comparatively inexpensive, the manufacturing costs of the brake application device, according to the invention, are not significantly affected. Furthermore, because of the low coefficient of friction of the slide bearing according to the invention, an extremely favorable efficiency of the lever mechanism is achieved so that the disk brake equipped with it has a very good hysteresis behavior.
Particularly advantageous characteristics of the bearing according to the invention were achieved when this bearing is lubricated. This type of a slide bearing should preferably have lubrication bore reliefs to further reduce the coefficients of friction and maintain it constant for a long time. In addition, even in the case of the slide bearing in the lubricated condition, a regular or multiple lubrication will not be necessary so that to this extent a lubrication for life is provided which therefore does not cause any additional shop costs.
When dimensioning the bearing surfaces, it should be endeavored according to the further development of the invention that the sliding surface of the slide bearing has such a dimension that the bearing stress occurring when the ball socket bearing is acted upon by maximum stress, amounts to at least 200 N/mm.sup.2, and preferably up to 300 N/mm.sup.2. Even after extreme long-time tests, these load values have resulted in no bearing failure and, because of the above-mentioned characteristic of the bearing that the coefficient of friction improves at an increased pressure, permit the adjustment of an optimal course of the hysteresis. During the dimensioning of the bearing, it should also be endeavored that the angle of wrap of the sliding bearing amounts to less than 120 degrees. This securely prevents any jamming of the ball socket bearing and thus of the brake application device.
An advantageous fastening of the slide bearing in the socket may be achieved, for example, by a cylindrical passage in the slide bearing which points toward the socket and which engages in a correspondingly shaped blind hole of the socket. Preferably, the cylindrical passage and the blind hole of the socket are dimensioned such that a press fit can be achieved. As a further advantage, the cylindrical passage is constructed in the center of the slide bearing.
In the case of the ball socket bearing according to the invention, the socket may be constructed either on the lever arm of the rotary lever or on the end of the plunger rod of the compressed-air cylinder. An essential aspect of the invention is in both cases, the ball of the ball socket bearing, in contrast to the state of the art, does not consist of a hardened hemisphere projection but of a roller bearing ball which is inserted or welded into a corresponding hemispherical recess of the plunger rod or of the lever arm. A roller bearing ball of this type is available as a standard component and, despite the high degree of hardness, is significantly less expensive than the implementation of a corresponding hardening process in the case of the hemisphere projection of the plunger rod in the case of the known application devices. Furthermore, a roller bearing ball of this type has almost ideal surface characteristics and an excellent roundness so that the bearing characteristics are improved further with respect to the efficiency and the hysteresis. The roller bearing ball according to the invention can be fastened in the hemispherical recess particularly advantageously by friction welding.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.